The spectral width, or bandwidth, of an optical signal in an optical communication system which propagates through a dispersive medium, such as optical fiber, is a crucial parameter. To minimize signal degradation caused by dispersion effects, the bandwidth of the optical signal should be as small as possible, and, preferably, limited by the information bandwidth of the data carried on the optical signal. See for example, G. P. Agrawal, Fiber-Optic Communications Systems 1992 (N.Y.: Wiley-lnterscience), pp. 46-57 and pp. 193-195. In many cases, the most inexpensive sources from which an optical signal is generated produce signals with spectral widths far exceeding that of the information bandwidth. For example, Fabry-Perot lasers with many spectral modes are inexpensive sources that have been used in experiments involving the QPSK data modulation format. See the article by S. L. Woodward and G. E. Bodeep, "Uncooled Fabry-Perot Lasers for QPSK Transmission." IEEE Photonics Technol. Lett., 1995, 7, pp 558-560.
Furthermore, in some proposed networks wavelength conversion in the distribution system would prove useful, for example in architecture where wavelength selective routing is desired. Generally, networks in which wavelength conversion are important, such as the MONET network, require all signals to have well controlled and narrow optical spectra. See, for example, R. E. Wagner, et al., "MONET: Multiwavelength Optical Networking," J. Lightwave Tech., 1996, 14, pp. 1349-1355.
As one example of an optical modulation format, subcarrier multiplexing is a popular low-cost method for transmitting digital signals. The QPSK modulation format is commonly used because it is very robust, yet uses bandwidth efficiently. See, for example, the article by G. R. Joyce and R. Olshansky, "Differential QPSK transmission system for 80 compressed digital video channels." OFC '93 Tech. Dig., Feb. 1993, pp. 224-225 and the article by S. L. Woodward and G. E. Bodeep, "Uncooled Fabry-Perot Lasers for QPSK Transmission." IEEE Photonics Technol. Lett.., 1995, 7, pp 558-560. Another example of an optical modulation format is simply digital intensity modulation or on-off keying (OOK).
It would therefore be desirable to be able to convert the wide, excessive spectrum of any optical source to a well-controlled, narrow band spectrum suitable for transmission over a link with dispersion.